Question

what is a diode and how does it work?

Answer 1

Diode is a two terminal device that allows current to flow in only one direction.
It contains an anode and a cathode.
This current that flow in the direction from anode to cathode.
The flow of charges forms current which makes it to work.

Answer 2

A diode is the simplest sort of a semiconductor device...it's a two end electronic component . it has low resistance of the flow of current in one end and high resistance in the other. 
 a semiconductor diode is the most common now-a-days..made up of crystalline
piece of semiconducting material and having two ends/ terminals on the front and the back.
 its applications are---> 
1)in radio demodulation--> The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts
2)power converters.
3)over-voltage protection
4)temperature measurements and a lot more.